The present invention relates to over tip leakage measurement and to tip clearance measurement. In particular it relates to over tip leakage measurement and to tip clearance measurement for a rotor arrangement, for example in a gas turbine engine.
In a rotor arrangement there is a hub with an annular array of blades around its periphery. Each blade has a tip at its radially outer extremity. The rotor is surrounded by a casing which is close to, but not abutting, the blade tips so that there is a small clearance defined between the casing and blade tips. It is advantageous to minimise the clearance, to improve efficiency of the rotor arrangement, whilst ensuring that some clearance remains so that there is no rubbing between the blade tips and casing. It is a necessary precursor to controlling the clearance to be able to accurately measure the tip clearance.
In a known arrangement, shown in FIG. 1, a proximity probe 2 is mounted through the casing 4. It is axially aligned with the tip of the rotor blade 6 to measure the clearance 8. The proximity probe 2 may be mounted to be recessed from the line of the casing 4 to prevent contact between the tips of the rotor blades 6 and the probe 2. Known probes 2 use capacitance, optics, microwave or eddy currents in order to determine the distance between the probe 2 and the tip of the rotor blade 6, and thus the clearance 8.
One disadvantage of the known arrangement is that mounting the probe 2 through the casing 4 reduces the mechanical strength of the casing 4 at this position. This may prevent the casing 4 from being suitable to contain a released rotor blade. A further disadvantage is that the thermal behaviour of the casing 4 in the vicinity of the proximity probe 2 is changed by the presence of the probe 2 which may cause the casing to distort out of circular during operation.
Another disadvantage of the known arrangement arises where the tips of the rotor blades 6 include features such as shrouds or other protrusions and recesses. In this situation any axial movement of the rotor blades 6, for example caused by thermal expansion of components in the axial direction, results in a different part of the tip being directly underneath the probe 2. Thus the distance measured by the probe 2, and thus the clearance 8 determined, will not necessarily be the tightest clearance 8 resulting from the radial position of the blade 6 relative to the casing 4.
A further disadvantage of the known arrangement is that the proximity probes 2 are complex and delicate. Therefore they are ill-suited to the harsh temperatures in which rotor arrangements, particularly those in gas turbine engines, often operate. The probes 2 of the known arrangement are thus unreliable.
A yet further disadvantage of the known arrangement is that the derivation of the clearance 8 is dependent on the previously measured or determined relative position of the probe 2 relative to the radially inner surface of the casing 4. This position can change, for example due to casing erosion, which results in an error in the derived clearance 8.